This invention relates generally to a temperature sensing system, and more particularly concerns a device in the form of a probe or modified probe having temperature sensors for deployment through an introducer sheath placed in a body lumen to allow retrograde delivery of the sensors for the measurement or monitoring of the core body temperature.
Under ordinary circumstances, thermoregulatory mechanisms exist in the healthy human body to maintain the body at a constant temperature of about 37° C. (98.6° F.), a condition sometimes referred to as normothermia. To maintain normothermia, the body's thermoregulatory mechanisms act to precisely balance the amount of heat generated by metabolic activity in the body with heat lost to the environment. For various reasons, however, a person may unintentionally develop a body temperature that is below normal, a condition known as hypothermia. In more recent times, hypothermia has been allowed or even induced for various therapeutic purposes.
Accidental hypothermia is generally a dangerous condition that may have serious medical consequences and may result from various conditions such as extreme exposure, injury, illness or anesthesia. Measures are usually taken to restore normothermia to a patient suffering accidental hypothermia. Simple methods for treating hypothermia include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. If the hypothermia is not too severe, these methods may be helpful. However, if the hypothermia is severe, and especially if the patient is undergoing surgery, such methods may be too slow, impractical and ineffective. One cannot wrap patients undergoing surgery in a warming blanket or immerse them in warm water, or ask severely hypothermic patients that may be unconscious, to swallow enough warm liquid to restore normothermia. Furthermore, where external control over body temperature is desired because the physician desires to induce and maintain hypothermia, these methods are generally not powerful enough to defeat the patient's thermoregulatory responses. For example, if a patient is cooled below the shivering threshold, generally about 35.5° C., the body will shiver and generate metabolic heat that will defeat the attempt to cool the patient to hypothermic levels. Even if the body's thermoregulatory responses are disabled by, for example, disease or anesthesia, surface cooling or warming methods are generally not powerful enough to provide control that can keep a patient at a particular temperature. If the patient begins to get too cold or to warm above the target temperature, the surface cooling and warming methods generally cannot react fast enough and with sufficient precision to maintain the target temperature.
Partly in response to the inadequacies of surface application of heat, methods have been developed for adding or removing heat to a patient's body by internal means. A patient being administered breathing gases, for example a patient under anesthesia, may have the breathing gases warmed. This method may be effective but is limited in the amount of heat that can be administered without injuring the lungs. Similarly, a patient receiving IV fluids may have the fluids warmed. This too may be effective in the case of mild hypothermia, but the temperature of the IV fluid is limited by the temperature that will be destructive to the blood, generally thought to be about 41° C.-49° C., and by the amount of fluid that is acceptable to administer to a patient.
A far more invasive method may be used to add heat to a patient's blood, particularly in the case of heart surgery. Blood is removed from a patient, circulated through a by-pass system, heated or cooled, and then reintroduced into the patient's body. This by-pass method is both fast and effective in adding or removing heat from a patient's blood, but has the disadvantage of involving a very invasive medical procedure which requires the use of complex equipment, a team of highly skilled operators, and is generally only available in a surgical setting, usually where the patient has his or her chest opened by a thorachotomy. It also involves mechanical pumping of blood and channeling the blood through various machines and external lines, all of which are generally very destructive of the blood tissue. Because of this, most surgeons avoid placing a patient on by-pass for greater than 4 hours, and if control of the patient's temperature is desired for longer than that time, this method is unavailable.
One method for adding or removing heat from a patient by adding or removing heat from the patient's blood that does not involve pumping the blood with an external, mechanical pump involves placing a heat exchange catheter in the patient's bloodstream and exchanging heat through the catheter. This endovascular temperature management (ETM) technique was described in U.S. Pat. No. 5,486,208 to Ginsburg, the complete disclosure of which is incorporated herein by reference. One method disclosed for doing so includes inserting a catheter having a heat exchange region comprising a balloon into the vasculature of a patient and circulating warm or cold heat exchange fluid through the balloon while the balloon is in the bloodstream.
In successful ETM, in addition to fast and precise changes in a patient's body temperature, fast and precise control over a patient's thermal condition is very desirable. A general apparatus and method of ETM control based on feedback from temperature probes in or on the patient is disclosed in U.S. Pat. No. 6,149,673 to Ginsburg, the complete disclosure of which is incorporated herein by reference. A similar method is described in PCT publication WO 00/10494 to Radiant Medical Inc., the complete disclosure of which is also incorporated herein by reference. In such methods, a signal representing the temperature of a target tissue, which in whole body ETM may be the core body temperature, is directed to a controller from a temperature probe inserted on or in the patient, and the controller then controls the exchange of heat between the heat exchange catheter and the patient's blood flowing past that catheter. That in turn controls the temperature of the patient. With such a method, precise and rapid control is dependent to a large extent on accurate temperature measurement of the target tissue and thus dependent on an accurate temperature probe located at an appropriate site.
Currently, the patient's temperature may be measured by any one of several generally available temperature probes. These include, for example, skin temperature probes, oral thermometers, tympanic probes that may be placed in the ear canal and perhaps even in physical contact with the ear drum, esophageal probes including nasoesophageal probes, rectal probes, bladder probes, temperature sensors placed on an insertion sheath, and temperature probes that may be inserted by needle directly into the target tissue. These may be highly accurate temperature probes for their purpose. However, when used to provide a temperature signal for ETM, each of these probes suffers from significant shortcomings.
Some probes may not give an accurate temperature of the target tissue. For example, if the target is the core temperature of the patient, a skin temperature is generally not an accurate representation of the core temperature; if cardiac muscle is the target tissue, a bladder probe might not be a sufficiently accurate measure of the temperature of that tissue. This is especially true when used in the context of changing temperature, for example when hypothermia is being rapidly induced by cooling a normothermic patient.
For example, lowering the heart temperature to 32° C. may be very beneficial for a heart attack victim, but lowering the temperature to 28° C. might lead to dangerous arrhythmias. A rectal temperature probe is generally very slow to respond to temperature changes in the body's core temperature, and thus if the target tissue is the heart, and the core temperature is being lowered quickly, a controller receiving its temperature signal from a rectal probe might not receive a temperature measurement that represents the current cardiac temperature and thus might continue cooling even after the cardiac tissue has reached a target temperature and the patient's actual cardiac temperature might dangerously overshoot the target temperature of 32° C. and drop the cardiac temperature below 28° C. In similar manner, probes placed in the bladder also tend to lag core body temperature when that temperature is being changed, i.e., when the patient is being cooled or warmed.
Some probes are awkward and too difficult to use. For example, tympanic probes are difficult to place and tend to fall out of the ear during use. Bladder probes are difficult and awkward to place and generally require a slow but constant flow of uring to function accurately. Rectal probes are inconvenient to use, especially where a sterile surgical field is required. A needle probed placed through a hypodermic syringe into the target tissue may be more accurate and precise but would require injecting the probe directly into the patient and may also require radioscopic or fluoroscopic confirmation of placement, procedures that are not always readily available. Such a procedure would also entail a risk to the patient and the discomfort of a needle stick into the target tissue which might be deep within the body.
Where a temperature probe is controlling an ETM procedure and thus is in or on a patient at the same time as an ETM heat exchange catheter, the probe may be unacceptably influenced by the temperature of the catheter and not accurately reflect the temperature of the target tissue, especially if the probe is located too close to the heat exchange catheter. Temperature probes or sensors placed on the insertion sheath, for example, tend to be unduly influenced by the temperature of the heat exchange catheter placed through the sheath. When the probe is placed in the vasculature at a location some distance away from the catheter so as not to be influenced by the catheter, however, it generally requires a second needle stick or incision, and may utilize a vascular site on the patient that is needed by a physician for some other purpose. For example, if the ETM catheter is located in the left femoral vein, and the probe is placed in the right femoral vein, it would require a separate stick, that is, a puncture of the vessel, for the probe and would make it difficult for an interventionalist to perform angioplasty from either the right or the left femoral artery. A temperature probe might be placed through the same introducer sheath used by an ETM heat exchange catheter to access the central vasculature, but in such a case it would generally be lying alongside the catheter and be influenced by the temperature of the catheter. If the heat exchange catheter had a central working lumen as described in the patents and publication described above, and was located in a central vein, for example the Inferior Vena Cava (IVC), a temperature probe might be passed through the working lumen and distal of the catheter to measure the temperature in the blood. Such a probe would not require a second stick to place it into the bloodstream; however, in this configuration the temperature probe would measure the temperature of the blood soon after it passed over the heat exchange surface and thus might not be an accurate measurement of the temperature of a target tissue or organ or a patient's core. In some cases, If the temperature probe is advanced far enough beyond the catheter tip to obtain an accurate measure, it may need to be positioned in or near the heart which could have serious health repercussions. Such a positioning of the probe would also generally require the use of fluoroscopy or x-ray, procedures which are not always available or desirable.
There is a need therefore, especially in the context of ETM which requires accurate temperature information of a patient's target tissue, for a temperature probe that is not unduly influenced by the temperature of the heat exchange catheter, is located to accurately reflect changes in the patient's temperature, may be conveniently placed, will not require that the patient endure additional punctures or surgical procedures, will not usurp other needed surgical or interventional sites, and can be maintained in place throughout the procedure. The present invention fulfills those needs as well as others.